1. Field of the Disclosure
The present disclosure relates generally to a method and an apparatus for monitoring a downlink control channel in order to reduce power consumption of a user equipment in a wireless communication system.
2. Description of the Related Art
In order to meet wireless data traffic demands, which have increased since the commercialization of the 4th Generation (4G) communication system, efforts to develop an improved 5G communication system or a pre-5G communication system have been made. For this reason, the 5G communication system or the pre-5G communication system is called a beyond-4G-network-communication system or a post-LTE system.
In order to achieve a high data transmission rate, implementation of the 5G communication system in an ultrahigh frequency (mmWave) band (e.g., 60 GHz band) is being considered. In the 5G communication system, technologies such as beamforming, massive MIMO, Full Dimensional MIMO (FD-MIMO), array antenna, analog beamforming, and large scale antenna technologies are being discussed as means to mitigate propagation path loss in the ultrahigh frequency band and increase a propagation transmission distance.
Further, the 5G communication system has developed technologies such as an evolved small cell, an advanced small cell, a cloud Radio Access Network (cloud RAN), an ultra-dense network, Device-to-Device communication (D2D), a wireless backhaul, a moving network, cooperative communication, Coordinated Multi-Points (CoMP), and received interference cancellation to improve the system network.
In addition, the 5G system has developed Advanced Coding Modulation (ACM) schemes such as hybrid FSK and QAM Modulation (FQAM) and Sliding Window Superposition Coding (SWSC), and advanced access technologies such as Filter Bank Multi Carrier (FBMC), Non-orthogonal Multiple Access (NOMA), and Sparse Code Multiple Access (SCMA).
To support transmission of downlink and uplink transmission channels in a wireless communication system, downlink control information (DCI) related thereto may be used. In conventional Long Term Evolution (LTE), DCI is transmitted on a physical downlink control channel (PDCCH), which is a separate physical channel for downlink control. The PDCCH is transmitted in at each subframe in the system band. A single PDCCH carries a single DCI message. A plurality of user equipments (UEs) may be simultaneously scheduled in the downlink and the uplink and thus, multiple PDCCHs are simultaneously transmitted in each cell. A cell-specific reference signal (CRS), which is a common reference signal in a cell, may be used for decoding a PDCCH. The CRS is an always-on signal that is transmitted in each subframe in the band, and scrambling and resource mapping for the CRS changes based on a cell identity (ID). All UEs that monitor PDCCHs may perform channel estimation using a CRS, and may decode PDCCHs. The CRS is transmitted to all of the UEs by using a broadcasting scheme, and thus, UE-specific beamforming cannot be used. Therefore, a multi-antenna transmission scheme for a PDCCH in LTE may be limited to open-loop transmission diversity.
Unlike the conventional system, a 5th Generation (5G) wireless communication system will support services having a significantly short transmission delay and requiring high connection density, in addition to services that require high transmission speed. Therefore, a 5G communication system will provide various services using different transmission/reception schemes and having different transmission/reception parameters in a single system in order to satisfy various user requirements and services. The communication system should also be designed in consideration of forward compatibility, such that an added service is not restricted by the current system. For example, various services that use and, at the same time, support a scalable numerology for an inter-subcarrier interval, or have different transmission time intervals (TTIs) are serviced simultaneously in one system. Unlike the conventional LTE, time and frequency resources should be more flexibly utilized in 5G, e.g., to secure flexibility in designing a control channel.
A conventional PDCCH is unsatisfactory for securing flexibility, however, since the PDCCH is transmitted over the entire band and the size of a control region is set to be specific to a cell. Therefore, the 5G wireless communication system is being designed to include a control channel that is flexibly allocated according to various requirements of a service.
For example, in the frequency axis, a 5G downlink control channel is transmitted by setting a subband, as opposed to being transmitted over the entire band. In the time axis, the 5G downlink control channel is transmitted by setting the number of orthogonal frequency division multiplexing (OFDM) symbols, the sizes of which vary based on a TTI, a subframe length, a slot length, a mini-slot length, etc. From the perspective of a reference signal for decoding the 5G downlink control channel, a demodulation reference signal (DMRS) which may be transmitted only in a region where a downlink control channel is transmitted, may be used, as opposed to a conventionally used CRS that is transmitted over the entire band. Through the DMRS, forward compatibility and resource allocation flexibility may be secured.
The downlink control channel may have various formats. However, these different formats are not generally known in advance by a UE.
Also, the downlink control channel is transmitted using a resource from a set of time and frequency resources defined as a search space, and thus, the accurate time/frequency resources in which the downlink control channel is transmitted are not known in advance by the UE. Therefore, decoding of the downlink control channel by the UE is often based on blind decoding.
Blind decoding indicates an operation in which the UE performs downlink control channel decoding with respect to all possible downlink control channel formats and all possible combinations of time/frequency resources in a given search space. The downlink control channel may be transmitted at time intervals in which data scheduling and transmission is performed (e.g., a subframe interval, a slot interval, a mini-slot interval, etc.), and the UE may need to perform blind decoding. For example, for a PDCCH of an LTE system, a UE may perform blind decoding of a downlink control channel a maximum of 44 times for each component carrier. However, from the perspective of power consumption of the UE, this may be a big burden. Therefore, to reduce the power consumption of the UE attributable to blind decoding of a downlink control channel, there is a desire for a new scheme for monitoring a downlink control channel.